Electrical capacitors made of thin film dielectric material which is metallized on both sides are used in large quantities in electrical devices because of their reliability, their desirable electrical characteristics and their relatively low cost. U.S. Pat. No. 2,470,826 issued on May 24, 1949 to W. McMahon illustrates a capacitor in which a double sided metallized dielectric layer is folded an odd number of times in a longitudinal direction. Another double sided pleated metallized dielectric layer capacitor is disclosed in U.S. Pat. No. 3,854,075 issued Dec. 10, 1974, to John Phillip Uhl, and assigned to the assignee of the present invention. In the Uhl patent, a capacitor is disclosed which is formed by pleating a double side metallized dielectric layer an even number of times and then winding the structure about the leads which act as mandrels and are left in the structure. Spraying of the ends of the wound capacitor structure is, therefore, not needed or contemplated by the Uhl patent. The McMahon patent suggests that his dielectric material may then be would along its longitudinal dimension after pleating and that end terminations may then be applied to a wound capacitor segment by spraying metal against the edges of the wound capacitor segment. The types of capacitor envisioned by the McMahon and Uhl patents, if commercially practicable, would be extremely desirable because substantially all of the dielectric layers of these capacitors are in the electric field and these capacitors may be made without the insertion of an additional dielectric layer in the main body of the capacitor, although a short initial unmetallized dielectric strip and a protective terminating dielectric strip could be employed with these capacitors, if desired.
The types of capacitor construction that were suggested in the McMahon and Uhl patents have a number of decided manufacturing and electrical advantages. For example, the problem of masking the dielectric layer during electrode evaporation when the electrode areas are applied to the film is substantially reduced. In addition, no slitting is required as it is during the manufacture of conventional wound film capacitors. Furthermore, the capacitance per unit dielectric area is maximized because of the substantially full use of the dielectric layer in the electric field. Shorter winding lengths per unit capacitance are thereby required, reducing the winding labor. The dielectric lengths can also be calibrated and pre-cut to produce a capacitor of more accurate values. Another advantage of these types of capacitors, if they can be made in a reliable manner, is that there are no air layers between the metallized electrodes and the pleated film surfaces and this substantially increases the voltage, for example, one hundred volts or more, at which corona discharge starts, making these capacitors commercially attractive for new applications such as fluorescent lighting circuits.
However, the construction of commercially successful capacitors as suggested in the McMahon and Uhl patents was hampered by problems that are presented by the attachment of leads to the wound capacitor segments. The McMahon patent suggested that leads could be attached to his capacitor by the well-known Schoop process. The application of high velocity metallic spray coatings to the edges of the capacitor described by McMahon is not commercially practical, however, because the spray will penetrate through the dielectric area at a pleat and thereby short one electrode to the other. If the temperature and pressure of the spray are lowered, penetration of the dielectric layers may be reduced but the adhesion of the metallic spray will generally be very poor. The multiple leads suggested in the McMahon patent also are not satisfactory since they tend to tear the dielectric material and to make the capacitor bulky and inductive.
In order to solve the above mentioned spray shorting problem at a pleated edge, it has been suggested in Australian Pat. No. 159,958, patented on Nov. 24, 1954 to provide unmetallized dielectric strips behind all of the pleats of a pleated metallized capacitor. While this solution may provide a technically feasible method of attaching leads to a pleated metallized dielectric layer capacitor, the number of unmetallized strips that are required on the dielectric layer greatly complicates the manufacturing process for both the dielectric layer itself and the pleated capacitor product. For example, in a capacitor having five pleats, there must be, in addition to the two unmetallized margin areas, five unmetallized strips that run along the entire length of the dielectric material, with three of these strips being on one side of the dielectric layer and two of these strips being on the opposite side of the dielectric layer. It is apparent that the masking problems and the cost of manufacturing such a specialized item make the solution of the Australian patent commercially unattractive. This is especially true since the type of metallized dielectric layer that is used in the capacitor of the present invention, (in which only the margin areas are required), is readily commercially available.
The leads of the capacitor of the Uhl patent were used as mandrels, as previously mentioned, to wind the pleated capacitor segment about the center, in a manner similar to that suggested in U.S. Pat. No. 2,887,649 issued May 19, 1959 to Daniel B. Peck. In the Uhl patent, conductive foil tabs were inserted between the leads and the metallized electrode areas to make positive contact with the electrode and to protect the electrode from damage by the leads during winding. However, it has been determined that this construction leads to a higher failure rate than desirable due to opening of the electrode area around the periphery of the metal foil, for reasons which are not completely understood.
A pleated capacitor having an even number of pleats which may be wound about its center may be achieved in accordance with the present invention whereby lead attachment is provided in a more reliable manner by the application of a metallized spray to the ends of the wound capacitor segment. The previously mentioned Peck patent suggested that metallized end spray could be used to attach leads to a double-sided metallized dielectric layer that was wound about its center. However, the Peck patent did not involve the pleating of the dielectric layer before winding and consequently the electrode areas were exposed to the spray only at their edges. The area of the thin film electrode edges that were exposed to the metallized spray was, therfore, so very small that it was virtually impossible to achieve adequate adhesion of the spray to the edges of the electrodes under commercially acceptable manufacturing processes.
The configuration of the capacitor of the present invention provides a product which has all of the advantages of pleated, or pleated and wound, capacitors and also has the added advantage that it may be manufactured with commercially available metallized dielectric material and wound about the center of the pleated dielectric layer by developed winding techniques. A further advantage of the present invention is that the same basic starting material may be used to manufacture capacitors having a number of different pleating combinations, which, of course, is not true for the capacitor of the Australian patent, in which each pleat combination requires a specially manufactured dielectric layer. In addition to the use of the present invention to manufacture wound film capacitors, it is also possible to utilize the invention to produce ceramic capacitors in which the capacitor dielectric material is folded while the ceramic material is still in a "green" state, in a manner similar to that disclosed in U.S. Pat. No. 3,223,494 issued Dec. 14, 1965 to J. W. Crownover.